Implement missing `xsave` intrinsics

[gnzlbg]

x86

• fn _xsave (p: *mut c_void, m: u64) -> ()
• fn _xrstor (p: *mut c_void, m: u64) -> ()
• fn _xsetbv (a: u32, v: u64) -> ()
• fn _xgetbv (a: u32) -> u64

x86_64

• fn _xsave64 (p: *mut c_void, m: u64) -> ()
• fn _xrstor64 (p: *mut c_void, m: u64) -> ()